The efficiency of singlet oxygen generation by substituted naphthalenes in benzene. Evidence for the participation of charge-transfer interactions

An inverse correlation between the rate constant for oxygen quenching of the triplet state (kTq) and the efficiency of singlet oxygen generation (SΔ) is established for a range of substituted naphthalenes in benzene. The participation of charge-transfer interactions are implicated on the basis that the only property which varies significantly with the substituent is the oxidation potential (EoxM) of the naphthalene derivative. Using published electrochemical data, correlations between SΔ, kTq and the free energy change for charge transfer (ΔGCT) have been found with values of SΔ and kTq ranging from 0.34 to 0.75 and from 5.0 × 109 to 1.2 × 109 M−1 s−1 for 1-methoxynaphthalene and 1-cyanonaphthalene, respectively.